Norseman,
You've started to answer your own question. Think along the lines of duvet covers (nylon?), night clothes and human movement during the night. Result, nice build up of charge. My suggestion would be to examine anything that has changed since before you started to have the problem. You may just be able to pin down the cause from this. In my opinion (for what that's worth!) you can discount any connection between the static and your household wiring system.
Having being involved in this subject in industry I know that the causes and cures (or limitation) for static is a very weird topic to get into. It may be of some comfort to you that each human reacts in a different way to electrical charges. Some people are very prone to feeling the smallest charge while others feel very little. Sadly, I am one of the first group (not ideal for someone working in the industry), the slightest charge can do me a serious mischief. The only consolation that you have is that it is a nuisance rather than danger factor in domestic situations.
Sorry not to be able to pin it down to a definiate cause.
Regards.